This invention relates to a drum brake device that functions as a leading-trailing (LT) type when the service brake is applied, and as a duo-servo (DS) type when the parking brake is applied. More specifically, it relates to a drum brake device in which the configuration effectively reduces the noise created when just releasing the parking brake.
This type of drum brake device has been disclosed, for example, in U.S. Pat. No. 5,275,260 and in Japanese Official Gazette No. 62-8652.
Its configuration will be explained with reference to FIG. 10 and FIG. 11. A wheel cylinder b and an anchor block c are set on the upper and lower segments respectively of a back plate a. The upper and lower ends of a pair of left and right brake shoes d, e are functionally engaged with the wheel cylinder b and the anchor block c, respectively.
Shoe return springs f, g are stretched between the upper and lower ends of the brake shoes d, e respectively. A parking lever h is pivoted on the lower segment of the left brake shoe d. A pivot lever i is pivoted such that its central segment can rotates on the central segment of the right brake shoe e. A rod j is mounted horizontally between the brake shoes d, e in the vicinity of the anchor block c. The left end of the rod j is engaged with the brake shoe d and the parking lever h, and the right end of the rod j is engaged with the brake shoe e and the lower segment of the pivot lever i, respectively.
On the wheel cylinder b side of the device, a screw-type shoe clearance adjustment device k is mounted horizontally between the brake shoes d, e. The left end of the shoe clearance adjustment device k is engaged with the brake shoe d, and the right end is engaged with the brake shoe e and the upper segment of the pivot lever i.
The above-explained drum brake device functions as a LT type brake by frictionally engaging with the brake drum n since both brake shoes d, e open with the point of abutment against the anchor block c as the fulcrum by operating the wheel cylinder b when the service brake is applied.
When the parking brake is applied, the rotational force of the parking lever h is transferred in sequence to the rod j, the pivot lever i, and the shoe clearance adjustment device k, whereupon the left shoe d opens, with its point of abutment with the anchor block c as the fulcrum, and frictionally engages with the brake drum n. Then, the pivot lever i opens, with its point of abutment with the shoe clearance adjustment device k as the fulcrum, whereupon the right brake shoe e, on which the pivot lever i is pivoted, opens simultaneously to frictionally engage with the brake drum n. Also, as in FIG. 11, the reaction force of the parking lever h effects on the lower left end of the brake shoe d.
As is shown in FIG. 11, at this point, should the vehicle be stopped on an incline or decline, and rotational force is applied on the brake drum n in the direction of arrow R, the friction force of the right brake shoe e is transferred via the shoe clearance adjustment device k as a force to extend the left brake shoe d. Should the force be applied in the opposite direction on the brake drum n, the friction force of the left brake shoe d is transferred via the shoe clearance adjustment device k to the right brake shoe e. As such, when the parking brake is applied, this drum brake device functions as a DS type brake.
When applying the parking brake on a level road, the right brake shoe e with the pivot lever i is separated from the anchor block c. If the parking brake is released under this condition, the right brake shoe e hits the anchor block c to create a noise, which gives an unsecure and uncomfortable feeling to the driver.
When the service brake is applied to stop on a slope and is released while the vehicle is stopping, the impact noise is created since the right brake shoe e rotates with the brake drum n until abutting against the anchor block c due to the rotation of the drum brake n in the opposite direction of arrow R as in FIG. 11.
Further, when the parking brake only is applied while stopping on a slope, either one of the brake shoes d or e separates from the anchor block c. If the parking brake is released under this condition when starting, the similar impact noise is created.